This invention relates to a process for preparing cyclosiloxanes, more particularly a process for reacting cyclosiloxanes with a rearrangement catalyst to yield higher molecular weight oligomers.
In a publication authored by Chojnowski, J.; Mazurek, M.; Scibiorek, M.; Wilczek, L., "Cationic Polymerization of Siloxanes" Die Macromolekulare Chemie 175, pp. 3299-3303, 1974, there is described the polymerization of hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane in the presence of protonic or Lewis acids. It is theorized by these authors that higher molecular weight polymers are formed by attaching cyclic monomers to the end of a growing macromolecular chain, which according to these authors, is consistent with the conventional understanding of cationic ring-opening polymerization reactions. They indicated that the structure of the active center of propagation and the way it reacts with the monomer is not clear, but oxonium ion, silicenium (silylenium) ion and free radical structures are postulated. The catalysts used in these polymerization reactions were CF.sub.3 SO.sub.3 H for both hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane and C.sub.3 H.sub.5 O.sub.2 SbF.sub.6 for octamethylcyclotetrasiloxane. Negligible amounts of higher oligomers were detected in this work.
In addition to the polymerization of cyclosiloxanes with catalysts, it has been known for some time that cyclosiloxanes containing more than four silicon atoms are produced in minute quantities during the commercial preparation of cyclosiloxane oligomers. These higher molecular weight cyclosiloxane oligomers having more than four silicon atoms can be isolated from lower molecular weight polymer mixtures having three or four silicon atoms only through laborious separation techniques; consequently if the yield of higher molecular weight polymers were increased the result would be an increase in yield efficiency after separation from the lower molecular weight polymers. Thus, it is beneficial to the art to have available a more efficient and economical process for producing higher quantities of cyclosiloxanes having more than four silicon atoms.
An advantage of the instant invention is the utilization of specific rearrangement catalysts to improve the efficiency of the reactions currently employed to produce higher molecular weight cyclic siloxanes.
Another advantage of the invention is the capability to selectively increase the yield of a specific molecular weight cyclosiloxane.